Is $20,000 a hard number. What if you make $20000 this year and would like a raise. That would automatically be detrimental, forcing you to either not take it until that raise is >20% over $20k. Then since is rent money, and you would have to income taxes on this additional money over $20k you would actually need 20% + 15% over 20k just to break even on what you were saving in rent.